Method of producing a control system



OCt- 7, 1969 G. c. BRElTwElsER 3,470,609

METHOD OF PRODUCING A CONTROL SYSTEM Filed Aug. 18, 1967 United StatesPatent O 3,470,609 METHOD 0F PRODUCING A CONTROL SYSTE Gary C.Breitweiser, St. Louis, Mo., assignor to Conductron Corporation, St.Charles, Mo., a corporation of Delaware Filed Aug. 18, 1967, Ser. No.661,586 Int. Cl. B01j 17/00;H01l1l/14 U.S. Cl. 29-571 8 Claims ABSTRACTOF THE DISCLOSURE An insulated-gate field-effect device can be renderedsubstantially free from the effects of migrating ions in the dielectriclayer thereof by applying a second dielectric layer over the gateelectrode of that insulated-gate field-effect device, by applying anauxiliary electrode over that second dielectric layer, by applying aD.C. voltage to the resulting multi-layer composite structure while thatmulti-layer composite structure is held at an elevated temperature tocause temperature-freed ions in the dielectric layers thereof to drifttoward that auxiliary electrode, and by permitting that multi-layercomposite structure to cool and substantially immobilize thosetemperature-freed ions, thereby keeping those temperature-freed ions outof the eld between the gate electrode and the semi-conductor of thatinsulated-gate field-effect device.

This invention relates to improvements in control systems. Moreparticularly, this invention relates to improvements in insulated-gatefield-effect devices and in methods of making same.

lt is, therefore, an object of the present invention to provide animproved insulated-gate field-effect device and an improved method ofmaking same.

An insulated-gate field-effect devicecustomarily includes a layer ofsemi-conductor, ohmic electrodes that are spaced apart but that areconnected by that layer of semi-conductor, dielectric layer thatoverlies the layer of semi-conductor and the ohmic electrodes, and agate electrode which overlies that dielectric layer. The application ofa varying voltage to that gate and to that layer of semi-conductor willvary the electrical characteristics of that portion of the layer ofsemi-conductor which interconnects the ohmic electrodes, and thus willcontrol the amount of current flowing between those ohmic electrodes.While insulated-gate field-effect devices have been found to be useful,many of those insulated-gate held-effect devices have not operatedproperly; because some of the ions in the dielectric layers between thelayers of semi-conductor and the gate electrodes have tended Y tomigrate and to adversely effect the electrical properties of thoseinsulated-gate held-effect devices. The present invention provides aninsulated-gate field-effect device which is substantially free fromchanges in the electrical properties thereof due to the migration ofions in the dielectric layer between the gate Velectrode and the layerof semiconductor thereof; and hence it provides an insulated-gatefield-effect device which is substantially free from failure due to ionmigration. lt is, therefore, an object of the present invention toprovide an insulatedgate iield-effect device which is substantially freefrom changes in the electrical properties thereof due to ion migrationin the dielectric layer between the gate electrode and the layer ofsemi-conductor thereof.

The present invention forms a second dielectric layer over the gateelectrode of an insulated-gate field-effect device and forms anauxiliary electrode over that second dielectric layer. A D.C. voltage isapplied to that auxiliary electrode and also to the layer ofsemi-conductor of the resulting multi-layer composite structure; and thetem- ICC perature of that multi-layer composite structure is elevated toa level at which temperature-freed ions in the dielectric layers thereofare rendered mobile. Those ions will largely drift out of the dielectriclayer between the gate electrode and the layer of semi-conductor andinto the second dielectric layer; and the multi-layer compositestructure will then be permitted to cool to substantially immobilizethose temperature-freed ions in that second dielectric layer. Thetemperature to which the multi-layer composite structure is elevatedduring the drifting of the ions is much higher than the temperatures atwhich the insulated-gate held-effect device will be operated; and hencethe substantially immobilized temperature-freed ions in the seconddielectric layer `will tend to remain out of the dielectric layerbetween the gate electrode and the layer of semi-conductor of theinsulated-gate fieldeffect device. As a result, that insulated-gatefield-effect device will be substantially free from changes in theelectrical properties thereof due to ion migration in the dielectriclayer between the gate electrode and the layer of semi-conductorthereof. It is, therefore, an object of the present invention to form asecond dielectric layer over the gate electrode and dielectric layer ofan insulatedgate field-effect device, to apply an auxiliary electrodeover that second dielectric layer, to apply a D.C. voltage to theresulting multi-layer composite structure while elevating thetemperature of that multi-layer composite structure to a level at whichtemperature-freed ions will drift from the dielectric layer of thatinsulated-gate fieldeffect device into that second dielectric layer, andto permit that multi-layer composite structure to cool down and therebysubstantially immobilize the temperature-freed ions in that seconddielectric layer.

Other and further objects and advantages of the present invention shouldbecome apparent from an examination of the drawing and accompanyingdescription.

In the drawing and accompanying description a preferred embodiment ofthe present invention is shown and described but it is to be understoodthat the drawing and accompanying description are for the purpose ofillustration only and do not limit the invention and that the inventionwill be defined by the appended claims.

ln the drawing, FIG. l is a plan View of one preferred embodiment ofinsulated-gate field-effect device that is made in accordance with theprinciples and teachings of the present invention,

FIG. 2 is a sectional view, on a larger scale, through part of theinsulated-gate field-effect device shown in FIG. 1, and it is takenalong the plane indicated by the line 22 in FIG. l, and

FIG. 3 is another sectional view through the insulatedgate field-effectdevice shown in FIG. l, it is on the scale of FIG. 2, and it is takenalong the plane indicated by the line 3-3 in FIG. 2.

Referring to the drawing in detail, the numeral 10 denotes a layer ofsemi-conductor; and,l in one preferred embodiment of insulated-gatefield-effect device that is made in accordance with the principles andteachings of the present invention, that layer is made of P typesilicon. The numeral 12 denotes an electrode of N type silicon which isdiffused into the upper surface of the layer 10; and the numeral 14denotes a further electrode of N type silicon which has been diffusedinto the upper surface of the layer 10 at a point spaced from the.contact 12. In the said one preferred embodiment of insulated-gateeldeffect device, the confronting portions of the electrodes 12 and 14are spaced apart about ve ten-thousandths of an inch. One of thoseelectrodes can serve as the source electrode of the insulated-gatefield-effect device; and the other of those electrodes can serve as thedrain electrode of that insulated-gate field-effect device.

The numeral 18 denotes a dielectric layer which overlies the electrodes12 and 14 and overlies portions of the layer 10. In the said onepreferred embodiment of insulated-gate field-effect device, that layeris silicon dioxide; and it is formed by oxidizing the upper surface ofthe layer and the upper surfaces of the electrodes 12 and 14. Thatdielectric layer is about four millionths of an inch thick. Openings 19and 21 are formed in that dielectric layer in register with theelectrodes 12 and 14; and those openings can easily be formed by anetching process.

The numeral denotes a metal grid which overlies part of the dielectriclayer 18, overlies the confronting portions of the electrodes 12 and 14,and overlies the portion of the layer 10 between those confrontingportions of those electrodes. That metal grid will constitute the gateelectrode of the insulated-gate field-effect device shown in FIG. 1. Thedielectric layer 18 and the metal grid 20 can be formed in the samemanner as the dielectric layer and gate electrode of a standardinsulated-gate field-effect device; and that dielectric layer can beidentical to the dielectric layer of such an insulated-gate held-effectdevice. However, the metal grid 20 is formed so it is open in nature,whereas the gate electrode of a standar-d insulatedgate eld-eiect deviceis formed so it is imperforate in nature. As a lresult, the layer 10,the electrodes 12 and 14, the dielectric layer 18, and the metal grid 20constitute an insulated-gate, iield-eifect device which can be similarto any one of a number of insulate-gate field-effect devices that are onthe market.

The numeral 22 denotes a dielectric layer which overlies the metal grid20, which overlies part of the dielectric layer 18, which overlies theconfronting portions of the electrodes 12 and 14, and which overlies theportion of the layer 10 between those confronting portions of thoseelectrodes. That dielectric layer will preferably have a thicknesscomparable to the thickness of the dielectric layer 18. While the layer22 should be a dielectric layer, the dielectric properties of that layerneed not be identical, or even closely similar, to the dielectricproperties of the dielectric layer 18. A primary requirement of thedielectric layer 22 is that the ions therein have a low level ofmobility in the range of temperatures at which the insulatedgateeld-eifect device will 4be operated. The dielectric layer 22 could bemade of silicon dioxide, silicon monoxide, silicon nitride,phospho-silicate glass, titanium dioxide, tantalum oxide, or the like;but silicon nitride is particularly desirable, because in siliconnitride the ions have a low level of mobility in the range oftemperatures at which insulated-gate field-effect devices normally areoperated.

The numeral 24 denotes a metal electrode which overlies part of thedielectric layer 22, which overlies the metal grid 20, which overliespart of the dielectric layer 18, which overlies the confronting portionsof the electrodes 12 and 14, and which overlies the portion of the layer10 between those confronting portions of those electrodes. That metalelectrode is electrically spaced from the metal grid 20 by thedielectric layer 22. The numeral '25 denotes a metal lead which isconnected to the layer 10, the numeral 26 denotes a metal lead which isconnected to the electrode 12 through the opening 19, the numeral 27denotes a metal lead which is connected to the electrode 14 through theopening 21, the numeral 28 denotes a lead-receiving portion of the metalgrid 20, and the numeral 29 denotes a lead-receiving portion of the`metal electrode 24. The metal leads 26 and 27 can be formed by a vacuummetallization process; and the rest of the insulated-gate field-effectdevice will be suitably masked during that vacuum metallization process.

The lead-'receiving portion 29 of the metal electrode 24 lwill beconnected to the negative terminal of a source of D.C. voltage, and themetal lead will be connected to the positive terminal of that source ofD.C. voltage. That source of DJC. voltage will provide a voltage whichshould be between one hundred thousand volts per centi- 4 meter of thecombined thicknesses of the dielectric layers 18 and 22 and a valuesomewhat below the values at which those combined dielectric layers willbreak down. Where the dielectric layers 18 and 22 have a combinedthickness of about ten millionths of an inch, the minimum value of theD.C. voltage applied to the lead 25 and the lead-receiving portion 29will be in the order of two to three volts. The multi-layer compositestructure which includes the layer 10, the electrodes 12 and 14, thedielectric layer 18, the metal grid 20, the dielectric layer 22, and themetal electrode 24 will be heated to a temperature in the range of onehundred to three hundred and fty degrees centigrade; and the saidvoltage will be applied to the lead 25 and to the lead-receiving portion29 to develop an electric eld across that multi-layer compositestructure. In one preferred embodiment of the present invention, avoltage of six volts was applied between the lead 2S and thelead-receiving portion 29, and the temperature of the multi-layercomposite structure was raised to two hundred and fifty degreescentigrade. That temperature and that voltage were maintained for onehour.

At temperatures in the range of one hundred to three hundred and fiftydegrees centigrade, many of the ions in the dielectric layers 18 and 22will become mobile; and those ions will respond to the electric fielddeveloped across the multi-layer composite structure to drift toward anoppositely-chargedlead or electrode. In the said preferredembodiment ofthe present invention, many of the positive ions which are in thedielectric layers 18 and 22 and which are in register with the metalelectrode 24 will drift toward the interface between that metalelectrade and the dielectric layer 22. Such drifting of those positiveions will increase the concentration of those ions in the dielectriclayer 22 and will correspondingly decrease the concentration of thoseions in the dielectric layer 18. At a temperature of two hundred andiifty degrees centigrade, the positive ions within the dielectric layers18 and 22 will be relatively quite mobile; and, within one hour, theD.C. voltage that is applied to the lead 25 and to the lead-receivingportion 29 of the metal electrode 24 will cause substantially all of thetemperature-freed positive ions that are in the dielectric layer 18 andthat are in register with the metal electrode 24 to drift successivelyto the interface between the dielectric layers 18 and 22, through thatinterface, and then through the dielectric layer 22 to the interfacebetween that dielectric layer and the metal electrode 24. The opennature of the metal grid 20 will enable temperature-freed ions to passthrough that metal grid as they flow from the dielectric layer 18 intothe dielectric layer 22, and thus will enable substantially all of thetemperature-freed ions in all portions of the dielectric layer 18 thatare overlain by the dielectric layer 22 to flow into the latterdielectric layer. At the end of that one hour, the multilayer compositestructure will be permitted to cool to room temperatures; and,thereupon, the temperature-freed positive ions in the dielectric layers18 and 22 will become substantially immobile. l

During the cooling period, the voltage can be applied to the lead 25 andto the lead-receiving portion 29 continuously, or disconnected 'fromthat lead and from that lead-receiving portion. Continued application ofthat voltage to that lead and to that lead-receiving portion isdesirable; because the higher-than-normal conceneration of positive ionsin the upper part of the layer 22 will cause those positive ions torepel each other, and thus will tend to cause some of those positiveions to drift back toward the dielectric layer 18. Where the cooling ofthe multilayer composite structure occurs at a relatively rapid rate,the drifting of those positive ions, due to the higher-thannormalconcentration of positive ions in the upper part of the dielectric layer22, will be small enough to be acceptable. However, by continuing toapply the D C. voltage to the lead 25 and to the lead-receiving portion29, all such drifting of the positive ions can be prevented.

After the temperature of the multi-layer, composite structure has fallento room temperature levels, the ability of the positive ions in thedielectric layer 22 to drift will be very limited; because thosepositive ions are substantially immobile at room temperatures. Moreover,because those positive ions will be outside of the dielectric layerdeveloped between the layer and the metal grid 20, which functions asthe gate electrode of the insulatedgate field-effect device, thatelectric field will not tend to cause those positive ions to try todrift back into the dielectric layer 18. The overall result is that thedielectric layer 18 will have substantially no mobile positive ionstherein; and hence the insulated-gate field-effect device of FIGS. 1-3will be substantially free from failure due to the migration of positiveions, in that dielectric layer, during the operation of thatinsulated-gate fieldeffect device.

In the said preferred embodiment of the present invention, theelectrodes 12 land 14 are diffused electrodes in the upper surface ofthe layer 10. However, it should be understood that the presentinvention is usable in making thin film, insulated-gate field-effectdevices; and, in such insulated-gate field-effect devices, theelectrodes 12 and 14 will be thin metal layers. It should also beunderstood that, if desired, a thin film, insulated-gate field-effectdevice could be formed so the layers thereof were inverted.Specifically, a thin film, insulated-gate field-effect device could beformed with the metal electrode 24 on the bottom, with the dielectriclayer 22 overlying that metal electrode, with the metal grid overlyingthat dielectric layer, with the dielectric layer 18 overlying that metalgrid, and with the thin film electrodes and the layer 10 ofsemi-conductor overlying the latter dielectric layer--those thin filmelectrodes overlying or underlying that layer of semi-conductor, asdesired.

In the said preferred embodiment of the present invention, the layer 10is a layer of P type semi-conductor. However, it should be understoodthat the present invention is usable in making insulated-gatefield-effect devices wherein the layer 10 is an N type semi-conductor.In the said preferred embodiment of the present invention, theinsulated-gate field-effect device is an enhancement-type insulated-gatefield-effect device. However, it should be understood that the presentinvention is usable in making depletion-type insulated-gate field-effectdevices. Further, it should be understood that the present invention isusable in making insulated-gate field-effect devices which have pluralgate electrodes. In making insulated-gate field-effect devices, such asdepletion-type insulated-gate field-effect devices, wherein NP junctionsare present in the layer of semi-conductor, it will be advantageous toconnect the one terminal of the source of D.C. voltage to both of theelectrodes 26 and 27 or to both of the corresponding thin filmelectrodes rather than to the electrode 25. Where that is done, theelectric field developed by that source of D.C. voltage will notadversely affect, and will not be rendered ineffective by, those NPjunctions.

If desired, the metal electrode 24 can be removed after the multi-layer,composite structure has cooled,down to room temperature levels. Theremoval of that metal electrode will not affect the higher-than-normalconcentration of temperature-freed ions in the upper surface of thedielectric layer 22, iand will not affect the lower-than-normalconcentration of temperature-freed ions in the dielectric layer 18.However, it should be recognized that it is not necessary to remove thatmetal electrode; because that metal electrode will not be in theelectric field that will be established between the metal grid 20 andthe layer 10 of the insulated-gate field-effect device. In anyinstallation wherein the presence of the metal electrode 24 might tendto develop an undesired capacitive effect, the lead which is connectedto the metal grid 20 could be connected to the lead which was connectedto the leadreceiving portion 29 of that metal electrode-such connectionessentially eliminating any such capacitive effect.

Where the dielectric layer 22 is made from a material that is differentfrom the material of which the dielectric layer 18 is made, there can bean increased resistance to the drifting of temperature-freed ions fromthe dielectric layer 18 into the dielectric layer 22. However, thatincreased resistance to ion drift will not, where the temperature of themulti-layer, composite structure is higher than one hundred degreescentigrade and where that temperature is maintained longer than an hour,be able to prevent the drifting of substantially all of thetemperature-freed positive ions in the dielectric layer 18 out of thatdielectric layer and into the dielectric layer 22. While the D.C.voltage is being applied to the lead 25 and to the lead-receivingportion 29 of the metal electrode 24, the electrodes 12 and 14 and themetal grid will usually be disconnected from any source of voltage.However, after the D.C. voltage is disconnected from the lead 25 and thelead-receiving portion 29 of the metal electrode 24, the electrodes 12and 14 and the metal grid 20 will be connectable to an appropriatecircuit.

Whereas the drawing and accompanying description have shown anddescribed a preferred embodiment of the present invention, it should beapparent to those skilled in the art that various changes may be made inthe form of the invention without affecting the scope thereof.

What I claim is:

1. The method of reducing ion migration within an insulated-gatefield-effect device, that has a layer of semiconductor and a dielectriclayer and an electrode, which comprises overlying at least a portion ofthe electrode and at least a portion of the dielectric layer of saidinsulatedgate field-effect device with a second dielectric layer, saidsecond dielectric layer abutting said portion of said dielectric layerof said insulated-gate field-effect device, heating the resultingmulti-layer composite structure and applying a D C. voltage to saidsecond dielectric layer and to the layer of semi-conductor of saidinsulated-gate fieldeffect device to cause temperature-freed ions insaid dielectric layer of said insulated-gate field-effect device todrift into said second dielectric layer, and subsequently cooling saidmulti-layer composite structure to substantially immobilize saidtemperature-freed ions in said second dielectric layer, whereby thepercentage of temperature-freed ions in said dielectric layer of saidinsulatedgate field-effect device is lower than normal.

2. The method of reducing lion migration within an insulated-gatefield-effect device, that has a layer of semiconductor and a dielectriclayer and an electrode, which comprises overlying at least a portion ofthe electrode and at least a portion of the dielectric layer of saidinsulatedgate field-effect device with a second dielectric layer, saidsecond dielectric layer abutting said portion of said dielectric layerof said insulated-gate field-effect device, heating the resultingmulti-layer composite structure and applying a D.C. voltage to saidsecond dielectric layer and to the layer of semi-conductor of saidinsulated-gate field-effect device to cause temperature-freed ions insaid dielectric layer of said insulated-gate field-effect device todrift into said second dielectric layer, and subsequently cooling saidmulti-layer composite structure to substantially immobilize saidtemperature-freed ions in said second dielectric layer, whereby thepercentage of temperature-freed ions in said dielectric layer of saidinsulatedgate field-effect device is lower than normal, said D C.voltage being applied to said second dielectric layer and to said layerof semi-conductor of said insulated-gate fieldeffect device so it has apolarity which makes said second dielectric layer negative relative tosaid layer 0f semiconductor, said D.C. voltage also making said seconddielectric layer negative relative to said dielectric layer of saidinsulated-gate field-effect device.

3. The method of reducing ion migration within an insulated-gatefield-effect device, that has a layer of semiconductor and a dielectriclayer and an electrode, which comprises overlying at least a portion ofthe electrode and at least a portion of the dielectric layer of saidinsulated-gate field-effect device with a second dielectric layer, saidsecond dielectric layer abutting said portion of said dielectric layerof said insulated-gate held-effect device, heating the resultingmulti-layer composite structure and applying a D C. voltage to saidsecond dielectric layer and to the layer of semi-conductor of saidinsulated-gate eld-eifect device to cause temperature-freed ions in saiddielectric layer of said insulated-gate eld-etect device to drift intosaid second dielectric layer, and subsequently cooling said multi-layercomposite structure to substantially immobilize said temperature-freedions in said second dielectric layer, whereby the percentage oftemperature-freed ions in said dielectric layer of said insulatedgatefield-effect device is lower than normal, said temperature being in therange of one hundred to three hundred and fifty degrees centigrade.

4. The method of reducing ion migration within an insulated-gatefield-effect device that has a layer of semiconductor and a dielectriclayer and an electrode, which comprises overlying at least a portion of`the electrode and at least a portion of the dielectric layer of saidinsulated-gate held-effect device with a second dielectric layer, saidsecond dielectric layer abutting said portion of said dielectric layerof said insulated-gate field-effect device, heating the resultingmulti-layer composite structure and applying a D.C. voltage to saidsecond dielectric layer and to the layer of semi-conductor of saidinsulated-gate field-effect device to cause temperature-freed ions insaid dielectric layer of said insulated-gate held-effect device to driftinto said second dielectric layer, and subsequently cooling saidmulti-layer composite structure to substantially immobilize saidtemperature-freed ions in said second dielectric layer, whereby thepercentage of temperature-freed ions in said dielectric layer of saidinsulatedgate feld-eiect device is lower than normal, said D.C. voltagehaving a value between one hundred thousand volts per centimeter of thecombined thicknesses of said dielectric layers and a value somewhat lessthan the value at which those combined dielectric layers will breakdown.

5. The method of reducing ion migration within an insulated-gatefield-effect device, that has a layer of semiconductor and a dielectriclayer and an electrode, which comprises overlying at least a portion ofthe electrode and at least a portion of the dielectric layer of saidinsulated-gate eld-eiect device with a second dielectric layer, saidsecond dielectric layer abutting said portion of said dielectric layerof said insulated-gate field-effect device, heating the resultingmulti-layer composite structure and applying a D.C. voltage to saidsecond dielectric layer and to the layer of semi-conductor of saidinsulated-gate held-effect device to cause temperature-freed ions insaid dielectric layer of said insulated-gate field-effect device todrift into said second dielectric layer, and subsequently cooling saidmulti-layer composite structure to substantially immobilize saidtemperature-freed ions in said second dielectric layer, whereby thepercentage of temperature-freed ions in Said dielectric layer of saidinsulatedgate iield-elr'ect device is lower than normal, saidtemperature being in the range of one hundred to three hundred and ftydegrees centigrade and said voltage having a value between one hundredthousand volts per centimeter of the combined thicknesses of saiddielectric layers and a value somewhat less than the value at whichthose combined dielectric layers will break down.

6. The method of reducing ion migration within an insulated-gatefield-effect device, that has a layer of semiconductor and a dielectriclayer and an electrode, which comprises overlying at least a portion ofthe electrode and at least a portion of the dielectric layer of saidinsulated-gate field-effect device with a second dielectric layer, saidsecond dielectric layer abutting said portion of said dielectric layerof said insulated-gate held-effect device, heating the resultingmulti-layer composite structure and applying a D.C. voltage to saidsecond dielectric layer and to the layer of semi-conductor of saidinsulated-gate field-effect device to cause temperature-freed ions insaid dielectric layer of said insulated-gate eld-eifect device to driftinto said second dielectric layer, and subsequently cooling saidmulti-layer composite structure to substantially immobilize saidtemperature-freed ionsin said second dielectric layer, whereby thepercentage of temperature-freed ions in said dielectric layer of saidinsulatedgate eld-eiect device is lower than normal, electrodes engagingsaid layer of semi-conductor and underlying said dielectric layer ofsaid insulated-gate field-effect device, and forming a further electrodeso it overlies said second dielectric layer and is in register with theconfronting portions of said electrodes.

7. The method of reducing ion migration within an insulated-gatefield-effect device, that has a layer of semiconductor and a dielectriclayer and an electrode, which comprises overlying at least a portion ofthe electrode and at least a portion of the dielectric layer of saidinsulated-gate field-elfect device with a second dielectric layer, saidsecond dielectric layer abutting said portion of said dielectric layerof said insulated-gate field-effect device, heating the resultingmulti-layer cornposite structure and applying a D.C. voltage to saidsecond dielectric layer and to the layer of semi-conductor of saidinsulated-gate iield-efect device to cause temperature-freed ions insaid dielectric layer of said insulatedgate field-effect device to driftinto said second dielectric layer, and subsequently cooling saidmulti-layer cornposite structure to substantially immobilize saidtemperature-freed ions in said Second dielectric layer, whereby thepercentage of temperature-freed ions in said dielectric layer of saidinsulated-gate ield-elect device is lower than normal, making saidelectrode of said insulated-gate eld-elect device open in nature sotemperature-freed ions can flow through it as they flow from said firstsaid dielectric layer into said second dielectric layer in response tosaid D.C. voltage.

8. The method of reducing ion migration within an insulated-gatefield-eiect device, that has a layer of semiconductor and a dielectriclayer and an electrode, which comprises overlying at least a portion ofthe electrode and at least a portion of the dielectric layer of saidinsulated-gate held-effect device with a second dielectric layer, saidsecond dielectric layer abutting said portion of said dielectric layerof said insulated-gate field-effect device, heating the resultingmulti-layer composite structure and applying a D C. voltage to saidsecond dielectric layer and to the layer of semi-conductor of saidinsulated-gate field-effect device to cause temperature-freed ions insaid dielectric layer of said insulated-gate fieldeffect device to driftinto said second dielectric layer, and subsequently cooling saidmulti-layer composite structure to substantially immobilize saidtemperature-freed ions in said second dielectric layer, whereby thepercentage of temperature-freed ions in said dielectric layer of saidinsulated-gate field-effect device is lower than normal, forming afurther electrode at the outer surface on said second dielectric layer,and applying said D.C. voltage to said second dielectric layer via saidfurther electrode.

References Cited UNITED STATES PATENTS PAUL M, COHEN, Primary ExaminerU.S. C1. XR.

